The growth and survival of androgen-dependent and castration-resistant prostate cancer (PCa) are driven by androgen receptor (AR) signaling which is largely modulated by AR co-activators. Inactivation of AR co- activators is emerging as a promising therapy for PCa. We reported Arrest-defective-1 protein (ARD1), an acetyltransferase, is a novel AR co-activator that acetylates AR for AR activation and PCa development; however, the clinical relevance and mechanisms of ARD1 regulation of AR in prostate tumorigenesis are unknown. In this application, we aim to determine the mechanisms of ARD1 regulation of AR signaling, the pathological impacts of AR acetylation on prostate tumorigenesis, and the clinical significance of ARD1 overexpression in PCa. Our preliminary study of a small cohort of PCa indicates that ARD1 overexpression correlates with PCa Gleason score. In addition, we have collected 707 PCa and prepared tumor tissue microarrays for this analysis. Moreover, we narrowed down the lysine residue acetylated by ARD1 to a 22- amino-acid motif located in the DNA binding domain (DBD) of AR. Furthermore, we show that ARD1 promotes AR nuclear translocation. In light of these data, we hypothesize that ARD1 overexpression may correlate with Gleason score and other clinical features of PCa by acetylating the specific lysine in AR for AR nuclear translocation, target gene expression, and prostate tumorigenesis. In this study, we proposed the following Specific Aims. 1. Determine the lysine in AR acetylated by ARD1 and its impact on AR activation and prostate tumorigenesis. We will first determine which one of the three lysines in the 22-amino-acid motif is acetylated by ARD1 by site-directed mutagenesis, in vitro acetylation assays, and mass spectrometry analysis. Then, we will determine the biological and pathological impacts of the acetylated residue on AR transcriptional activity, PCa cell proliferation, and xenograft tumor growth using constitutively acetylated or non-acetylated AR mutants. 2. Determine the mechanism of ARD1-acetylated AR in prostate tumorigenesis. Preliminary data shows that ARD1 promotes AR nuclear translocation. We will determine if the lysine acetylated by ARD1 is required for AR nuclear translocation and AR-targeted promoter binding using the constitutively acetylated or non-acetylated AR mutants and chromatin-immunoprecipitation (ChIP) assays. 3. Determine the clinical correlation between ARD1 expression levels and PCa characteristics. We will perform immunohistochemistry analysis of the 707 PCa tissue microarrays using an exclusive ARD1 antibody. The correlation of ARD1 levels with PCa clinical features will be determined with assistance from a pathologist and a biostatistician. These experiments will enhance our understanding of the clinical significance of ARD1 overexpression in PCa progression and the mechanisms of ARD1 in AR signaling and prostate tumorigenesis which will be critical for future development of ARD1-AR signaling-dependent therapeutic approaches to manage and treat PCa.